#include<stdio.h>
int main()
{
	double x,y;
	scanf("%lf",&x);
	if(x>=0&&x<5)
	y=-x+2.5;
	else
	y=x<10?2-1.5*(x-3)*(x-3):x/2-1.5;
	printf("%.3f",y);
    return 0;
}

